1. Field of the Invention
This invention relates to a thermoplastic resin film permitting excellent paper feed and discharge and excellent transfer of offset printing ink and ink adhesion even under high humidity conditions in the summertime.
2. Description of the Related Arts
An opaque or semitransparent film comprising a single polyolefin film layer containing a fine inorganic powder or a laminate having such a fine inorganic powder-containing polyolefin film layer on at least one side thereof, especially a microporous stretched film, is useful as synthetic paper [JP-B-46-407994 (the term "JP-B" as used herein means an "examined published Japanese patent publication"), U.S. Pat. No. 3,765,999).
Synthetic paper comprising the above-mentioned fine inorganic powder-containing or organic filler-containing thermoplastic resin film has experienced a steadily increasing use over a very broad field, for example, a poster paper, atlas paper, bar code paper, prepaid card base paper, in-mold labels, playing card paper or a support for a heat transfer recording paper, because of its excellent physical properties such as water resistance, toughness and surface smoothness. With its increasing utility, synthetic paper of this type having improved antistatic properties and printability in offset printing or printing with ultraviolet-curing inks is in greater demand than ever.
Because polyolefins per se are non polar and hydrophobic, synthetic paper, particularly, polyolefin synthetic paper does not always-have satisfactory antistatic properties or offset printability, and it is usually subjected to an appropriate surface treatment such as corona discharge treatment, plasma treatment or ozone treatment, to improve these properties.
Among such surface treatments, it is the common practice to apply primer coating with polyethyleneimine or the like subsequent to the corona discharge treatment (U.S. Pat. No. 4,906,526, U.S. Pat. No. 4,420,536, JP-B-43-6676). The above method is however accompanied by the drawbacks that uniform primer coating cannot be obtained, presumably because the corona discharge treatment is inferior in the capacity to make the surface hydrophilic, and adhesion of an offset ink to a film is inferior, presumably because molecular cleavage of the polyolefin resin occurs owing to corona discharge and low-molecular weight oxides are formed.